1. Field of the Invention
The present invention relates to a method and related apparatus for stereo vocal cancellation, and more particularly, to a method and related apparatus, which cancels vocals of different stereo signals respectively.
2. Description of the Prior Art
With advanced information and electronics technology, various types of entertainment systems are available. For example, karaoke systems can play background music filtered of vocals, allowing users to sing along and enjoy a professional entertainment environment. However, music provided at retail outlets generally includes vocals, so in order to meet the requirements of an accompaniment system vocal cancellation technology is used, which aims to attenuate the vocals and leave the background music intact.
Please refer to FIG. 1. FIG. 1 shows a block diagram of a player 10 that performs vocal cancellation according to the prior art. In general, modern systems can play two-channel (or more) stereo sound, with different speaker modules of the player outputting the different stereo signals to allow users to hear realistic sound. The player 10 has a sound source circuit 12 to provide two stereo signals (such as left and right stereo signals), a signal module 14 to perform vocal cancellation, and two speaker modules 16A, 16B to output the stereo sound. The sound source circuit 12 can be a CD reading mechanism, which utilizes a reading head 18 to access and data of a CD 20 to demodulate it. The CD 20 has music data of two stereo channels, so the sound source circuit 12 reads two stereo signals PLi, PRi after accessing the data of CD 20. The signal module 14 performs vocal cancellation for the stereo signals PLi, PRi to generate two output signals PLo, PRo. The speaker modules 16A, 16B have individual A/D converters, power amplifiers, and speakers to transform the output signals PLo, PRo into acoustic waves.
In order to perform vocal cancellation, the signal module 14 has two high pass modules 26A, 26B, a low pass module 28, and a vocal cancellation module 22. The high pass modules 26A, 26B high pass filter the stereo signals PLi, PRi to generate two corresponding high pass signals PLh, PRh; and the low pass module 28 filters a signal Ps to generate a corresponding low pass signal P1. The vocal cancellation module generates an intermediate signal PVC by the difference between the two stereo signals PLi, PRi. The output signal PLo is generated by mixing a sum of the high pass signal PLh corresponding to the stereo signal PLi, the low pass signal P1, and the intermediate signal PVC. The output signal PRo is generated by mixing a sum of the high pass signal PRh corresponding to the stereo signal PRi, the low pass signal P1, and the intermediate signal PVC.
To illustrate vocal cancellation of the mentioned prior art, please refer to FIG. 2. FIG. 2 shows the spectrum of each stereo signal, with each horizontal axis of the spectrum being frequency, and each vertical axis of the spectrum being amplitude.
Generally speaking, commercially produced music establishes stereo sound by mixing different signals of background music. The vocal track is mixed into each stereo signal with equal intensity. When a user plays the stereo signals with the speaker modules, they hear the vocals as being ahead because the components of the two stereo channels are equal. Different kinds of background music in each of the stereo channels makes the user hear the stereo effect, as if the background music is around the user. In FIG. 2, a spectrum Vf represents the vocal spectrum, and spectrums Lmf, Rmf individually represent the different spectrums of background music. As mentioned above, the sum of the background music spectrum Lmf and the vocal spectrum Vf generates a spectrum of a left stereo signal Lf, and the sum of the background music spectrum Rmf and the vocal spectrum Vf generates a spectrum of a right stereo signal Rf. Like the stereo signals PLi, PRi accessed from the sound source circuit 12 in FIG. 1, their spectrums may be shown as the spectrums Lf, Rf. Because of the physical limitation of the human voice, which produces vocals not below a specific low frequency or exceeding a specific high frequency, the vocal spectrum is usually limited to a specific bandwidth. The frequencies fl, fh denoted in FIG. 2 individually represent the lower bound and the upper bound of the human vocal spectrum. The vocal spectrum Vf is concentrated on the intermediate-frequency band BM between the frequencies fl and fh. In contrast to the vocal spectrum Vf limited to the intermediate-frequency band BM, the spectrum of background music produced by all kinds of musical instruments is of a broader bandwidth. Shown in FIG. 2, the spectrums Lmf, Rmf of background music spread into the low-frequency band BL below the frequency f1, and into the high-frequency band BH above the frequency fh. In addition to the intermediate-frequency band BM that the vocal spectrum is located in, each signal spectrum of the stereo channel Lf, Rf is also expanded into the low-frequency band BL and the high-frequency band BH.
Because each stereo signal has the same vocal signals, the signal module 14 (please refer to FIG. 1) subtracts the stereo signal PRi from the stereo signal PLi in the vocal cancellation module 22 to remove the common vocals of the two stereo signals and generate the intermediate signal PVC. The stereo signals PLi, PRi located in the low-frequency band BL and the high-frequency band BH are reduced in the subtraction process, and the vocal cancellation should contain the components of background music in the low-frequency band BL and the high-frequency band BH. Thus, the signal module 12 uses the high pass modules 26A, 26B and the low pass module 28 to perform high-frequency compensation and low-frequency compensation. The high pass module 26A extracts the components of the stereo signal PLi in the high frequency band BH to generate the high pass signal PLh. The signal source Ps of the low pass module 28 may be one of the stereo signals PLi, PRi. The low pass module 28 extracts the component of the signal Ps in the low-frequency band BL to generate the low pass signal P1. The mixing sum of the high pass signal PLh, the low pass signal Pl, and the intermediate signal PVC can compensate for the high and low-frequency components lost in the vocal cancellation to generate the output signal PLo.
In the same way, after the high pass module 26B extracts the high-frequency components of the stereo signal PRi to generate the high pass signal PRh, the signal module 12 can use the high pass signal PRh and the low pass signal P1 to perform high and low-frequency compensations for the intermediate signal PVC in order to generate the output signal PRo. In general, each stereo signal in the low-frequency band BL does not have direction, so it is difficult to build the stereo sound effect by the difference of the stereo signals PRi, PLi in the low-frequency band. Thus, the signal module 14 uses the same low-frequency signal P1 to perform low-frequency compensation for the output signals PLo, PRo. In contrast, each stereo signal in the high-frequency band BH has direction, and the difference of the stereo signals in the high-frequency band BH allows the user to hear the stereo sound effect. Thus, the signal module 14 individually uses the high pass signals PRh, PLh, high pass filtered by the two stereo signals PRi, PLi, to perform high pass compensation, as well as utilizing the difference of the output signals PRo, PLo in the high-frequency band to produce the stereo sound effect. In summary, the signal module 14 receives two stereo signals PLi, PRi, uses the vocal cancellation module 22 to generate the intermediate signal PVC as the essential result of vocal cancellation, uses the low pass signal Pl and the high pass signals PLh, PRh as the low and high-frequency compensations respectively, and individually generates the output signals PLo, PRo as the results of the stereo signals PLi, PRi after vocal cancellation. The signal module 12 attenuates the vocals of the two stereo signals while somewhat preserving the stereo sound effect of the background music in the output signals PLo, PRo.
Please refer to FIG. 3. FIG. 3 shows the spectrum when the signal module 14 of FIG. 1 operates. In FIG. 3, each horizontal axis is frequency and the vertical axis is magnitude. Continuing the spectrum example in FIG. 2, if the spectrums of the stereo signals PLi, PRi in FIG. 1 are the spectrums Lf, Rf in FIG. 2, after the signal module 14 operates the spectrums of the output signals PLo, PRo are as the spectrums PLof, PRof in FIG. 3. The frequencies fl, fh, the low-frequency band BL, the intermediate-frequency band BM, and the high-frequency band BH denoted in FIG. 3 are the same as those of FIG. 2. To compare the difference of two spectrums PLof and PRof, FIG. 3 illustrates the spectrum PRof with a dotted line and the spectrum PLof with a solid line.
Because the output signals PLo and PRo generated by the signal module 14 in FIG. 1 have the same intermediate signal PVC and the same low pass signal Pl, the different parts are the different high pass signals PLh, PRh for high pass compensation. Compared with the spectrums PLof, PRof of the output signals PLo, PRo in FIG. 3, the main difference is concentrated in the high-frequency band BH, the components of the intermediate-frequency band BM and the low-frequency band BL of two spectrums PLof, PRof being the same. Although the high-frequency components of the signals have the stereo sound effect, most of the energy of the spectrums PLof, PRof is concentrated in the intermediate-frequency and low-frequency bands BM, BL. The signal energy distributed in the high-frequency band BH is relatively low, which results in little difference between the spectrums PLof, PRof. As such, when the player 12 outputs the output signals PLo, PRo, the stereo sound effect is not as expected. This is one disadvantage of the prior art. In other words, in the prior signal module 14 of FIG. 1, the output signals PLo, PRo of the two stereo channels both use the same intermediate signal PVC as the essential signal of vocal cancellation, and only use the different high pass signals PLh, PRh as the high pass compensation. The difference of the output signals PLo, PRo in only concentrated in the high-frequency and less-energy portions, and is not significant enough to generate an obvious stereo effect. This reduces stereo sound effect quality, and lessens user enjoyment.